In 3rd Generation Partnership Project, “3GPP”, Long Term Evolution, “LTE”, networks, a dormant user terminal is known at the cell level and retains its Radio Resource Connection, “RRC”, and established radio bearers. However, a user terminal entering dormancy activates discontinuous reception/transmission, “DRX”, for power savings during periods of temporary inactivity. Because user terminals are quickly moved into the dormancy sub-state, quick reactivation of the user terminal is crucial for good system performance. Consequently, the 3GPP Release-10, also referred to as LTE-Advanced, target for the dormant to active transition is 10 ms, excluding the DRX cycle—see the 3GPP Technical Specification, “TS”, 36.913 for more information.
To achieve this activation time target, working groups associated with developing relevant portions of the LTE standard have proposed mechanisms for reducing user-plane latency. One approach involves the use of contention-based access techniques for reducing latency. See 3GPP TS 36.912 for example information.
Contention-based uplink transmissions allow network-synchronized user terminals to transmit uplink data without sending Scheduling Requests, SRs, in advance. Eliminating the need for SRs reduces both the latency and signaling overhead. Such an approach can be particularly advantageous for small transmissions where the overhead associated with securing dedicated resources is not warranted.
Fast dynamic assignment of uplink resources for use in making contention-based transmissions is achieved by using the Downlink Physical Control Channel, “PDCCH”, to send grants. By using the PDCCH, contention-based uplink grants can be assigned to unused uplink resources on a per sub-frame basis, so that scheduling of uplink contention-free transmissions is not affected. In this way, a static assignment of contention-based uplink resources can be avoided, and instead be dynamically assigned, depending on the uplink load.
Contention Based Radio Network Temporary Identifiers, “CB-RNTI”, are used to identify contention-based uplink grants on the PDCCH. The contention-based uplink grants can have the same format as used for Release-8 user equipments, UEs, where the grant information specifies the granted resource blocks, the Modulation and Coding Scheme, “MCS”, and Transport Format, “TF”, to be used for a given contention-based uplink transmission. Further, for the LTE context, a new format known to Release-10 user terminals may be used for making contention-based uplink grants.
However, with several users in a cell, collision probability becomes an issue for contention-based transmissions on the uplink. If several user terminals attempt a contention-based uplink transmission responsive to the same contention-based uplink grant—i.e., on the same set of uplink resources allocated by a contention-based uplink grant—a transmission collision results and it is likely that none of the transmissions will be successful. One approach to reducing the likelihood of collision involves signaling more than one contention-based uplink grant within a given sub-frame but such approaches correspondingly entail significant and undesirable increases in the signaling overhead.